Amphiphilic Phenyl-Ethynylene Polymers and Copolymers. Synthesis, Characterization, and Optical Emission Properties

A series of new rigid amphiphilic aryl-ethynylene homopolymers and copolymers with phenyl, pyridinyl, thionyl, or carbazolyl as aryl units has been synthesized. A single side chain on the phenyl rings (alkyl-hydroxyl ester group) provides an amphiphilic character to the polymer backbone. The polymers are characterized by NMR, elemental analysis and size exclusion chromatography (SEC); the thermal properties are analyzed by TGA, DSC, and X-ray diffraction. The polymers with side chains are enough amphiphilic to give stable Langmuir films on water, and the LB films can be transferred on hydrophilic or hydrophobic substrates. The films are deposited on treated glass or silicon substrate. Analyzed by grazing incidence X-ray analysis (GIXA) and by atomic force microscopy (AFM), they reveal well-structured layering periods of 3.5 and 2.7-2.9 nm for the phenyl homopolymer (pPEnBz) and the copolymers with pyridine (pPEn(Bz-co-Py)) and thiophene (pPEn(Bz-co-Ti)), respectively. These results suggest a rearrangement in a Y-type bilayer for the pPEnBz on a hydrophilic substrate. For copolymers pPEn(Bz-co-Py), and pPEn(Bz-co-Ti), deposited on a hydrophobic substrate, the self-assembly within one layer consists of interdigitated alkyl chains. In solid-state films the polymers and copolymers show a rather good photoluminescence yield and large Stokes-shifts with emission peaks at 539, 502, 569, and 563 nm for pPEnBz, pPEn(Bz-co-Py), pPEn(Bz-co-Ti), and pPEn(Bz-co-Cz), respectively. The photoluminescence quantum yields are decreasing in the series by 23, 15, 12, and 7% respectively, while no emission is observed from pPEnPy. A dichroism on LB films has been found in absorption (R = 2.0, 2.04 and 1.3) and in emission spectra (R = 3.4, 4.0 and 2.6) for pPEnBz, pPEn(Bz-co-Ti) and pPEn(Bz-co-Py), respectively. LED properties are demonstrated using the ITO/polymer/LiF-Al sandwich, yielding photon emission at 555 and 550 nm for pPEnBz and pPEn(Bz-co-Py). These results are very important for the future realization by LB deposition technique of more sophisticated devices for polarized emission with further enhanced brightness and improved light efficiency.